1. Field of the Invention
The invention is related to the field of Nuclear Magnetic Resonance (NMR) methods. In particular, the invention details new methods for creating excitation pulses using stochastic processes in measurement procedures in downhole wellbore logging techniques.
2. Description of the Related Art
NMR is used for well logging and MWD to obtain information on wellbore structure in the form of the NMR signal amplitude, and T1 and/or T2 relaxation times. From these measurements can be determined quantities that are of importance in oil drilling, such as total porosity, bound and unbound fluid, permeability, etc. The traditional method for this downhole NMR is pulsed NMR, using hard (strong) RF pulses, which cause spin-flip angles of the order of 90° and 180°. In a more general sense, this type of NMR is spectroscopy of the strong nonlinear type, meaning that the response of the NMR system is not proportional to the strength of the excitation.
NMR methods are based on the well-understood properties of nuclear spin moments subjected to both a static external magnetic field and an oscillating external magnetic field. It is known that in the absence of a magnetic field, nuclear spin vectors will orient themselves in random directions. In the presence of only a static magnetic field, these individual spins tend to align themselves along the direction of the applied field. This alignment gives rise to an overall magnetization, referred to as the bulk magnetization vector. When the external field is removed, the spins resume their random orientation, and the bulk magnetization vector falls to zero.
Typical NMR methods determine properties of the material by observing how applied RF magnetic fields affect the spin vectors. In current methods, spins are first aligned via the application of a static external magnetic field. In well logging, this magnetic field is typically provided by a permanent magnet. Once the spins have reached saturation, a single RF magnetic pulse is applied perpendicular to the static field which aligns the spins generally in the plane perpendicular to this applied field in a direction generally perpendicular to both the static and RF fields. This pulse is referred to as a 90° pulse. If only the 90° pulse is applied, measurements can be obtained for the rate at which the spins realign along the direction of the static magnetic field. The rate of return along this direction is characterized by a time constant known as the spin-lattice relaxation time, T1.
After the application and removal of a 90° magnetic pulse, the spins, as well as realigning along the direction of the constant field, exhibit a precession around the direction of the static field with a frequency known as the Larmor frequency. The Larmor frequency is given by ω0=γB0, where γ is the gyromagnetic ratio and B0 is the strength of the applied constant field.
Typically, the effective static magnetic field is inhomogeneous throughout the formation. As a result according to its local magnetic field, each spin vector tends to precess at slightly different rates. The phase between the vectors, originally nearly zero at the moment the RF magnetic field is removed, diffuses as some vectors spin faster and some spin slower. The diffusion of the phase leads to a reduction of the component of the bulk magnetization in the plane perpendicular to the applied field. This process is known as dephasing. This decay is known as the free induction decay rate and is characterized by its time constant, T2*. The dephasing can be recovered partly as long as the underlying cause, the local spatial variation of the magnetic field is static. This is done by using one or more refocusing pulse and leads to the formation of one or more spin echos. The decay of these echos is characterized by its time constant T2.
In prior art, in order to observe the values for these time constants, and in particular of T2, the practitioner often applies a sequence of RF magnetic pulses. A sequence of pulses that is used widely in current methods is known as the Carr-Purcell-Meiboom-Gill (CPMG) sequence. In this sequence, the first pulse is a 90° pulse, which aligns the spins generally perpendicular to the applied static magnetic field. Subsequent pulses have twice the duration of the first pulse, and as a result are able to flip each spin vector a full 180° from the direction it had immediately prior to the application of the pulse. A 180° pulse is typically applied during a dephasing stage of the spin echos. After the pulse is removed, the order of the spins is reversed, with the slowly precessing spins spatially in front of the faster precessing spins. The phase between the spin vectors, which was previously diffusing, now is converging back to zero. At convergence, the spin vectors are generally aligned in a common direction again, and the bulk magnetization vector reaches a maximum value, creating a magnetic pulse known as a spin echo. The spin echo induces a voltage in a receiver coil, which is measured through the electronic assembly attached to the coil.
The CPMG sequence can be expressed asTW−90−(t−180−t−echo)n  (1)where TW is a wait time, 90 is an excitation pulse having a tipping angle of 90°, and 180 is a 180° refocusing pulse. This gives a sequence of n echo signals. U.S. Pat. No. 6,163,153 to Reiderman et al and co-pending U.S. patent application Ser. No. 09/551,761 of Slade et al, both having the same assignee as the present application, disclose use of a modified CPMG sequence in which the refocusing pulses have tipping angles of less than 180°. This can significantly reduce the power requirements. However, power requirements even for tipping angles of 90° can still be significant. In addition, CPMG sequences or modified CPMG sequences such as those taught by Reiderman and by Slade are relatively inefficient in power utilization in that the duty cycle is small, i.e., large amounts of power are expended in relatively short time intervals and in much of the time (the time between the pulses), no power is used and the apparatus is waiting for the nuclear spin system to respond to the pulses. It would be desirable to have a method of NMR logging that can reduce the power requirements and make more uniform use of power, while still obtaining useful information about formation properties. The present invention satisfies this need.